Correcting Rydberg excitations in argon clusters: From trimer to excimer localisation

The effect of diabatisation is reported in the excited argon clusters using the Diatomic-In-Molecules (DIM) method. In a previous study using DIM, the lowest energy isomers of Ar N ∗ were shown as Ar 3 ∗ − Ar N − 3 , however, using the Hole-Particle Pseudopotential (HPP) method, it was shown that the lowest triplet excitation is localised over the dimer not trimer; Ar 2 ∗ − Ar N − 2 . In this study we improve the DIM calculations by including previously ignored strongly avoided crossing between 3p4s and 3p4p 1 , 3 Σ states. • Corrected diabatised DIM (Di-DIM) for Rydberg-excited argon clusters up to Ar 13 . • Shows lowest Rydberg isomers localize on Ar 2 ∗ excimers, not Ar 3 ∗ trimers. • Di-DIM excited-state energetics closely match CASPT2/HPP benchmarks for key isomers. • Explicit treatment of 3p4s–3p4p avoided crossings fixes DIM’s qualitative failures. • Provides a low-cost, diabatic excited-state Hamiltonian suitable for dynamics and ML data.